This invention relates to velocity error control apparatus and, more particularly, to such apparatus which is used in a time base corrector for video signals, wherein velocity errors which are included in a video signal that is read out from a video signal reproducing device, such as a video tape recorder (VTR) are effectively eliminated.
When video signals are recorded on a magnetic medium, such as magnetic tape by a VTR, time base, or frequency and/or phase errors may be introduced into the video signals during playback because of various factors. For example, the record medium may expand or contract after the video signals have been recorded thereon. Also, the speed with which the record medium is transported in the playback device may differ slightly from its speed during recording. Similarly, there may be a slight variation between the rotary speed at which the heads scan the record medium during signal recording and during signal playback. All of these possible variations result in time base errors which appears as undesired effects in the ultimately reproduced video picture. These observable effects are perceived as jitter, brightness distortion, improper color display, and the like. If the video signal which is played back from, for example, a VTR, is to be used in connection with the transmission of a television broadcast, it is important that these time base errors be corrected.
One example of a time base corrector for use with video signals is described in U.S. Pat. No. 3,860,952. In this time base corrector, incoming video signals are converted from analog form into digital form and are temporarily stored in a digital memory. The digitized video signals are written into the memory at a write-in clock rate which varies in accordance with detected time base errors. However, the stored, digitized video signals are read out at a standard, fixed read-out clock rate, whereby the read out video signals are generally free of significant time base errors. Then, the read out digitized video signals are reconverted back into analog form. However, this time base corrector does not take velocity errors into account.
The velocity error of a video signal is the time base error which may exist over a substantial portion of a horizontal line interval, which error is not detected until the completion of that line interval and, thus, is not taken into account, or corrected, during that line interval. That is, in typical time base error correcting systems, the write-in clock signal has its frequency synchronized with the actual horizontal synchronizing pulse which is included in the played back video signal, and its phase is synchronized with the phase of the burst signal which also is included in the played back video signal. In accordance with the NTSC standard for television signals, both the horizontal synchronizing pulse and the burst signal are provided at the beginning of a horizontal line interval. Consequently, the synchronizing of the write-in clock signals to the incoming horizontal synchronizing pulse and to the incoming burst signal necessarily is achieved at the beginning of the horizontal line interval. Of course, if there is a time base error, or velocity error, in the video information portion of the horizontal line interval, there will be a phase shift between the write-in clock signals and the video signals. Since the digitized video signals are read out from the memory in the time base corrector at a standard, fixed read-out clock rate, the aforementioned phase shift during the write-in operation is not compensated during the read-out operation because of the use of such a fixed read-out clock rate. Consequently, the resultant video signal which is read out from the memory in the time base corrector may include velocity errors which are not compensated.
In copending application Ser. No. 817,662, assigned to the assignee of the present invention, a time base corrector is disclosed which includes a velocity error detector and a velocity error correcting circuit. The velocity error is detected by comparing the phase of the write clock pulses, that is, the clock pulses which are used to write the digitized video signal into the memory of the time base corrector, to the phase of the burst signals which are played back from the VTR. Any phase differential therebetween represents the shift in phase during a horizontal line interval of the played back video signal. This phase shift representation is the velocity error and is stored in a velocity error memory device at a storage location corresponding to the storage location in the memory at which the line of video signals which includes such velocity error is stored. When the stored line of video signals is read out from the memory at the constant read-out rate, the velocity error which is associated with that line of video signals also is read out of the velocity error memory. This read out velocity error signal is integrated over a line interval so as to represent a linearly changing velocity error over that line interval. The integrated velocity error signal then is used to phase modulate the read-out clock pulses, whereby the line of video signals are read out of the memory at a phase-modulated rate. It is assumed that this phase modulation of the read-out clock pulses is a close approximation of the velocity error which had been included in that line of video signals originally; and, therefore, compensates for this velocity error.
However, in the aforementioned copending application, the velocity error in the incoming line of video signals is assumed to vary linearly throughout the entire line interval. That is, the velocity error is assumed to increase gradually from a zero error at the beginning of the line interval to a maximum error at the end of that line interval. In actuality, it is believed that the velocity error does not vary in this linear manner. Although the linear approximation will, in most instances, be a sufficient approximation so as to compensate for velocity error, and thus minimize distortion in the ultimately reproduced video picture, a more accurate approximation of the actual velocity error is desired.
Another example of a time base corrector including velocity error compensating circuitry is described in U.S. Pat. No. 4,065,787. In this patent, the velocity error signal is determined by subtracting the difference between the control voltage which is applied to a voltage-controlled oscillator in the write-in clock generator phase-lock loop when one line of video signals is received from the control voltage which had been applied to the voltage-controlled oscillator when the preceding line was received. This difference is stored in a velocity error memory location corresponding to the location in the memory at which the presently received line of video signals is stored, and is read out when that line of video signals also is read out. Here too, the read out velocity error signal is integrated so as to approximate the actual velocity error as a linear function. Accordingly, this linear approximation of the velocity error in U.S. Pat. No. 4,065,787 is less accurate than desired.
A similar velocity error compensating arrangement is described in the paper "Digital Time Base Correction" by Kitson, Fletcher and Spencer, International Broadcasting Convention Technical Paper, September 1974. This arrangement also suffers from the disadvantage in that the velocity error is approximated by a linear function, which is not a highly accurate approximation.